Several types of communications networks exist today, including, for example, computer networks such as wide area networks (WANs), metropolitan area networks (MANs), local area networks (LANs), and personal area networks (PANs), and cellular networks. User terminals may communicate with the networks wirelessly, e.g., through radio frequency (RF) connections or infrared (IF) connections. The interface between a network and such wireless user terminals is generally called an air interface. Interface devices exist on both sides of the air interface. The interface device in the user terminal may be a wireless adapter, a cellular phone, etc. The interface device in the network may be a base station, a femtocell base station, a home base station, a relay station, an access point, an access network, etc. A network generally contains multiple network interface devices, each covering a certain area called a cell and communicating with user terminals within that area.
Network interface devices and user terminals exchange data in the form of frames, which are transmitted at specified times with specified time intervals. A frame, or a radio frame, is a data unit upon which a network operates, and consists of a number of bits of information including user data and/or overhead information. With the common knowledge of frame configuration, i.e., the structure and timing of radio frames, network interface devices and user terminals can intelligently communicate with each other.
Typically, network interface devices in a network operate with the same frame structures and the same frame timing. FIG. 1 illustrates a conventional frame configuration in a network with four network interface devices NID1, NID2, NID3, NID4. The frames of network interface devices NID1, NID2, NID3, NID4 not only have the same size, but are substantially aligned to one another in time.
As shown in FIG. 1, each frame configuration includes a plurality of radio frames. Each radio frame may include a control portion and a data portion. Overhead information is transmitted in the control portion, and user data are transmitted in the data portion of a frame. The overhead information may include synchronization signals, such as preambles, mid-ambles, post-ambles, reference signals, etc., that allow user terminals and the network interface devices to synchronize and maintain synchronization with each other. The overhead information may also include system information such as the identity of the network interface device, service options supported by the network interface device, and system parameters, etc. The overhead information may also include resource allocation information, such as downlink map (DL-MAP), uplink map (UP-MAP), multicast and broadcast service map (MBS-MAP), etc., that provide uplink and downlink slot allocations within a radio frame. A network interface device may broadcast the overhead information such that all user terminals within its cell can receive such information.
A wireless communication system defined by the IEEE 802.16e standard has a frame configuration similar to the one shown in FIG. 1. In an IEEE 802.16e system, each radio frame is divided into two subframes. One of the subframes is used for uplink transmission and the other for downlink transmission. Uplink transmission refers to the transmission from a user terminal to the network interface device, and downlink transmission refers to the transmission from the network interface device to the user terminal.
Before a user terminal can communicate with a network, the user terminal may first detect a network interface device by searching for the overhead information in the control portions from that network interface device. The user terminal may synchronize to the network interface device using the synchronization signals in the control portions and obtain such other system information as system parameters, supported services, etc. Thereafter, the user terminal may establish a communication session with the network through the network interface device.
Once the communication session is established, the user terminal may use the overhead information in the control portion of the radio frames to maintain synchronization to the network interface device and to update system information pertinent to the communication session. A mobile user terminal may also evaluate signal strengths of several network interface devices by measuring the broadcast signals in the control portion of radio frames from these network interface devices. If the evaluation indicates that a network interface device other than the one currently communicating with the user terminal will be able to provide a better connection quality, the user terminal or the network may decide to continue the communication session through such other network interface device. This process of switching the communication session from one network interface device to another is commonly referred to as handover.
Oftentimes, handover is necessary when a mobile user terminal moves out of the cell of the network interface device currently in communication with the user terminal and into the cell of a neighboring network interface device. The network interface device with which the user terminal is currently communicating is referred to as a serving network interface device, and the neighboring network interface device may be referred to as a target network interface device. When the signal strength of the serving network interface device declines as the user terminal moves away from the serving network interface device, it may be desirable for the user terminal and the network to continue the communication session through the target network interface device.
Before handover can take place, the user terminal may determine the signal strength of network interface devices nearby the serving network interface device to identify a suitable target network interface device. Generally, the serving network interface device may schedule certain time intervals during which the communication between the network and the user terminal is temporarily suspended to enable user terminal to measure the signals of the neighboring network interface devices. The measurements are referred to hereinafter as handover measurements. Based on the measurement results, the user terminal, the serving network interface device, or the network determines whether a handover should take place.
A conventional handover procedure is now explained with reference to FIG. 2, which illustrates handover measurements defined in the IEEE 802.16e standard.
In IEEE 802.16e, the network interface devices are base stations, and the user terminals are mobile stations. When a mobile station (MS) needs to measure the signals from neighboring base stations in preparation for a handover, communication with the network through the serving base station may be temporarily suspended. The measurement results may be reported back to the serving base station. Based on the measurement results, the network, the serving base station, or the user terminal determines whether a handover should take place.
The MS may initiate the handover measurements by sending a request to the serving base station. Alternatively, the serving base station may issue a command to the MS to initiate the measurements. The period from the initiation of the handover measurements to the completion of all the measurements and necessary reports thereof is referred to as a handover measurement period.
In the example shown in FIG. 2, the handover measurements are initiated by the base station. The serving base station is base station BS1, and the neighboring base stations include base stations BS2 and BS3. BS1 issues a command “MOB_SCN-RSP” to the MS to instruct the MS to measure signals from the neighboring base stations including BS2 and BS3. (201.) The MOB_SCN-RSP command includes several parameters: “start frame,” “scanning interval,” “interleaving interval,” and “iteration.” “Start frame” specifies when the MS should start the measurements, “scanning interval” specifies how much time the measurements should take, “iteration” specifies how many scanning intervals are allocated for the measurements, and “interleaving interval” specifies the time interval between two adjacent scanning intervals. Normal communication between the MS and BS1 may be temporarily suspended during the scanning interval and resumes during the interleaving interval. If measurement results need to be reported, the report is submitted during the interleaving interval. “Start frame,” “scanning interval,” and “interleaving interval” are all specified in numbers of radio frames. In an IEEE 802.16e network, the size of a frame may be 5 milliseconds. In the example shown in FIG. 2, the MS should start the measurement at the M-th frame after the MS receives the MOB_SCN-RSP command, measure the signals from BS2 and BS3 within a scanning interval of N frames, resume communication with BS1 for P frames, and may measure additional neighboring base stations during the additional scanning intervals.
During the scanning interval, the MS detects the synchronization signals from BS2, synchronizes to BS2, and measures the signals from BS2. (202.) Then the MS waits for the synchronization signals from BS3 and repeats the same process for BS3. (203.) Because the frames of the base stations are substantially aligned, the wait time between the measurements of BS2 and BS3 is approximately a full radio frame, e.g., 5 milliseconds.
If the signals from a neighboring base station are weak, the MS does not need to report the measurement results to BS1. If the signals from a neighboring base station are strong, then a handover is possible and the MS may need to submit a report to BS1. A threshold signal strength may be set by the network for determining when a report needs to be submitted. FIG. 2 assumes that the strength of signals from at least one of BS2 and BS3 exceed the threshold, and the MS needs to report back to BS1.
At the beginning of the interleaving interval, normal communication between the MS and the network through BS1 resumes, and the MS sends a request to BS1 for additional bandwidth for submitting the report. (204.) Upon receiving allocation of resource (205) for the submission of the report, the MS sends the measurement results to BS1 using the allocated resource. (206.) The report will be used by the network or BS1 to determine whether the communication should continue through BS2 or BS3. Alternatively, the MS may determine based on the measurement results if a handover is desired, and include in the report a request for handover. Again, because the frames of the base stations are aligned, and also because the mobile station needs to obtain allocation of resources for reporting, the wait time between the completion of the measurements and the report will be more than one frame, or 5 milliseconds.
If multiple scanning intervals have been allocated, the MS enters into another scanning interval at the end of the interleaving interval to measure signals from additional neighboring base stations.
The overhead information, such as the synchronization signals and the system information, consumes resources that would otherwise be available for normal data transmission. It is desirable to minimize overhead information, i.e., to minimize the control portion of radio frames. For example, so-called superframes were introduced in the IEEE 802.16m standard to reduce the control portion relative to the data portion in a frame. FIG. 3 illustrates the configuration of the superframe defined in the IEEE 802.16m standard.
The top portion of FIG. 3 shows the frame configuration viewed from a base station conforming to the IEEE 802.16m standard. The base station is also backward compatible with the IEEE 802.16e standard. The base station uses two interleaved frames. One frame has the same size as a frame defined in the IEEE 802.16e standard and is labeled “802.16e Frame.” Another frame as defined in the IEEE 802.16m standard, referred to as and also labeled “Superframe,” includes four 5 ms “802.16m frames.” Each superframe begins with a superframe header (SFH) that may contain an 802.16m preamble and/or other system parameters. As shown in FIG. 3, the 802.16m frame and the 802.16e frame together make up the entire relevant time period, although separately they each comprise discontinuous time intervals. Also, the beginning of a superframe is offset with respect to an 802.16e frame.
Each of the 802.16m frame and the 802.16e frame includes a control portion and a data portion, respectively labeled “802.16m Control” or “802.16e Control” and “802.16m Data” or “802.16e Data.” The control portion of the 802.16e frame contains overhead information for the operations of the base station under the IEEE 802.16e standard. The data portion of the 802.16e frame is used for data communication between the base station and mobile stations operating under the IEEE 802.16e standard. The control portion of the 802.16m frame contains overhead information for the operations of the base station under the IEEE 802.16m standard. The data portion of the 802.16m frame is used for data communication between the base station and mobile stations operating under the IEEE 802.16m standard.
The middle portion of FIG. 3, labeled “802.16e MS,” illustrates the frame configuration viewed from a mobile station operating under the IEEE 802.16e standard. The bottom portion of FIG. 3, labeled “802.16m MS,” illustrates the frame configuration viewed from the perspective of a mobile station operating under the IEEE 802.16m standard. These frame configurations should be understood by one skilled in the art and therefore are not explained in detail herein.
Compared to the IEEE 802.16e frame, where a preamble appears every 5 milliseconds, the preamble in a superframe appears only every 20 milliseconds. Consequently, the superframe configuration defined in the IEEE 802.16m standard requires much less overhead information as compared to the frame configuration defined in the IEEE 802.16e standard, and therefore results in improved system efficiency.
A problem arises, however, when a mobile station needs to perform handover measurements in preparation of a handover. A mobile station conforming to the IEEE 802.16e standard expects to wait for no more than 5 milliseconds between measurements of two neighboring base stations. However, a mobile conforming to the IEEE 802.16m standard may need to wait for up to 20 milliseconds between measurements of the neighboring base stations. As a result, handover measurements may take a significantly longer time in an IEEE 802.16m network than in an IEEE 802.16e network. The longer wait may force call drops if the mobile station cannot timely establish connection to a target base station. Similarly, the delay from the completion of the measurements and the report thereof will be significantly longer in IEEE 802.16m networks than in IEEE 802.16e networks. Such a long delay may result in an inaccurate report and wrong handover decisions.